Electricity networks for the transmission of electrical power generally have a series of substations, which connect a very-high-voltage level, a high-voltage level, a medium-voltage level and a low-voltage level to one another. In the past, electrical power was produced centrally by power stations, as a result of which the direction of the power flow in the electricity networks was known, specifically from the high-voltage level to the low-voltage level. Until now, it has therefore been possible to assume that the medium voltage at the output of the substation is at the most between the high-voltage level and the medium-voltage level, and that this falls correspondingly in the direction of an input of a transforming station. With the increase in decentralized generating installations, the distribution networks have become subject to new requirements. Within the context of the exploitation of the renewable energy sources, very-small generating installations are increasingly being connected to the electricity network, producing electrical power which is fed into the medium-voltage level, in particular close to the transforming stations. As a result, load peaks can occur because of feedback, and can damage end-user appliances as a result of impermissible excessive voltages.
Furthermore, in general, the feeding behavior of very-small generating installations, that is to say the reliability and the amount of power produced, for example in the case of solar installations and the like, is unpredictable. In the future, it will be desirable to provide better monitoring for the power flow in the electricity network, for example, at the medium-voltage level. In order to obtain information about the power flows and the voltage profile in the medium-voltage networks without having to equip all the transforming stations with instrumentation, it will be necessary to know the medium voltage which is present on the input side at some transforming stations. However, without direct measurement at the transforming stations, the medium voltages which are present there cannot easily be derived.
Furthermore, no instrumentation is normally installed in existing transforming stations, by means of which it is possible to detect the medium voltage which is present on the input side at the transforming station. Because of the compact design of transforming stations, it is generally difficult to retrofit such voltage measurement on the input side.
Furthermore, in the case of existing installations, the no-load currents of the transformers in the transforming station are unknown, as a result of which it is not easily possible to calculate back the medium voltage which is present on the input side, on the basis of a voltage which can be measured easily and is therefore known, and a current which can be measured easily and is therefore known on the low-voltage side of the transformer. In addition, the transformer tap position, that is to say the transformation ratio of the transformer can occasionally be manually matched to changed medium voltages, in order to achieve the desired network voltage at the low-voltage level. However, automatic detection of a change in a transformer tap position is possible, but difficult. Manual readjustment is also problematic, since tap position changes are normally not carried out by the specialist personnel who are also trained for changing the configuration of the local instrumentation and automation.